Wavelength division demultiplexing apparatus

ABSTRACT

The invention provides a wavelength division demultiplexing apparatus which can reduce the connection loss between an input slab and channel waveguides and can suppress excitation of higher-order mode light to reduce the loss. The apparatus includes a first waveguide for propagating WDM light, a first slab for diffusing the light from the first waveguide, a plurality of channel waveguides having a series of different waveguide lengths with a predetermined difference for receiving and splitting the light from the first slab, a second slab for condensing the split light from the channel waveguides, and a second waveguide for propagating the light from the second slab therein, all formed on asubtrate. The channel waveguides and the first slab are optically connected to each other at a number of nodes greater than the number of nodes at which the channel waveguides and the second slab are connected to each other.

BACKGROUND OF THE INVENTION

[0001] 1) Field of the Invention

[0002] This invention relates to a wavelength division demultiplexingapparatus particularly suitable for use with a wavelength divisionmultiplexing and demultiplexing apparatus of the arrayed waveguidegrating (AWG) type, which is used for wavelength division multiplexcommunication.

[0003] 2) Description of the Related Art

[0004]FIG. 28 is a block diagram showing a configuration of a commonwavelength division multiplexing and demultiplexing apparatus of the AWGtype. The wavelength division multiplexing and demultiplexing apparatuscan function as any of a wavelength division multiplexing apparatus anda wavelength division demultiplexing apparatus. In the followingdescription, a wavelength division multiplexing and demultiplexingapparatus is referred to as MUX/DEMUX and is used as a term signifying awavelength division multiplexing apparatus or a wavelengthdemultiplexing apparatus unless otherwise specified. Further,description is given of a case wherein, taking notice principally of thedemultiplexing function from between the multiplexing function and thedemultiplexing function the MUX/DEMUX has, the MUX/DEMUX functions as awavelength division demultiplexing apparatus. It is to be noted that theinputting and outputting directions of light when the wavelengthdivision multiplexing function of the MUX/DEMUX operates are reverse tothose when the wavelength division demultiplexing function of theMUX/DEMUX operates.

[0005] Referring to FIG. 28, the MUX/DEMUX 106 shown includes a singleinput waveguide 101, an input slab 102, a plurality of channelwaveguides 103, an output slab 104, and n output waveguides 105 allformed on a substrate 100 such that the input waveguide 101, input slab102, channel waveguides 103, output slab 104 and output waveguides 105may have a relatively high refractive index or indexes when comparedwith that of a surrounding region 10A.

[0006] It is to be noted that, in the following description, a portionformed from a material which has a relatively high refractive index whencompared with that of the region 100A is sometimes referred to as“core”, and another portion formed from a material which has arelatively low refractive index and surrounding the core such as theregion 100A is sometimes referred to as “clad”. The input waveguide 1,input slab 2, channel waveguides 3, output slab 4 and output waveguide 5correspond to the core, and the region 100A surrounding the inputwaveguide 1, input slab 2, channel waveguides 3, output slab 4 andoutput waveguide 5 corresponds to the clad.

[0007] In the MUX/DEMUX 106 shown in FIG. 28, when light multiplexed ina wavelength region is inputted to the input waveguide 101 of theMUX/DEMUX 106, light split for different wavelengths is outputted fromchannels #1 to #n of the output waveguides 105. On the other hand, whenlight of a plurality of different wavelengths is inputted to thechannels #1 to #n of the output waveguides 105, light in which the lightof all of the wavelengths is bunched and multiplexed in a wavelengthregion is outputted from the input waveguide 101.

[0008] In the following, the configuration of the MUX/DEMUX 106 isdescribed in comparison with the configuration of a conventionalspectroscope (monochro-meter). The functions of the MUX/DEMUX 106 areimplemented, for example, by not only AWG type devices shown in FIGS. 28and 29(a) but also spectroscope type devices shown in FIGS. 35 and 29(b)and other devices.

[0009]FIG. 35 is a view showing an example of a configuration of aconventional spectroscope. Referring to FIG. 35, the spectroscope shownis of the bulk diffraction grating type, and it is generally difficultto reduce the pitch of a diffraction grating. In contrast, aspectroscope of the AWG type does not require the pitch, and it is onlynecessary to design the differences in length among waveguides whichcompose the AWG.

[0010] Meanwhile, FIG. 29(a) is a schematic view showing a core patternof the waveguides of the MUX/DEMUX 106 of the AWG type and particularlyshows core portions of the MUX/DEMUX 106. The components (elements orparts) 101 to 105 of the MUX/DEMUX 106 shown in FIG. 29(a) individuallycorrespond to components of a spectroscope.

[0011]FIG. 29(c) is a view illustrating a corresponding relationshipbetween the components of a wavelength division multiplexing anddemultiplexing apparatus configured using waveguides and a conventionalspectroscope. The corresponding relationship is described with referenceto FIG. 35. The spectroscope 110 shown in FIG. 35 includes, in additionto a diffraction grating 113 with an uneven or rough surface, a singleinput optical fiber 111, an input collimate lens 112, a condenser lens114, and n output optical fibers 115.

[0012] The input waveguide 101 which is a component of the MUX/DEMUX 106(refer to FIG. 29(a)) diffuses and outputs wavelength divisionmultiplexed laser light, which is an object of wavelength divisiondemultiplexing, to the input slab 102 in the following stage. Further,as seen in FIG. 29(c), the input waveguide 101 functionally correspondsto the input optical fiber 111 of the spectroscope 110 in that it has arole of an incidence slit for spreading light. It is to be noted thatFIG. 29(a) is a schematic view particularly showing core elements in theMUX/DEMUX 106.

[0013] Similarly, the input slab 102 diffuses light incoming to theinput waveguide 101 and couples the diffused light to the channelwaveguide 103 in the following stage. The input slab 102 corresponds toa function of the input collimate lens 112 in the spectroscope 110 (afunction of aligning incoming light powers from the input optical fiber111 and irradiating them upon the diffraction grating 113 in thefollowing stage).

[0014] Meanwhile, the channel waveguides 103 which correspond to thediffraction grating 113 of the spectroscope 110 deflect light to apredetermined angle for each of wavelengths as hereinafter described,and the output slab 104 which corresponds to the condenser lens 114condenses the lights outputted (outgoing or radiated) from anddiffracted by the channel waveguides 103. The output waveguides 105which correspond to the output optical fibers 115 cut part of a spectrumof the light outgoing from the output slab 104.

[0015] Here, the channel waveguides 103 are formed with differentlengths such that the channel waveguide at the lowermost position of theMUX/DEMUX 106 shown in FIGS. 28 and 29(a) has the smallest length andany other channel waveguide at a higher position has a successivelyincreasing length. The differences in length between adjacent ones ofthe channel waveguides are equal to one another. The channel waveguidesperform significant operation in wavelength division (splitting of lightfor each wavelength) or wavelength division multiplexing. In thefollowing, operation of the channel waveguides 103 is described.

[0016] FIGS. 30(a) and 30(b) are views showing three neighboring channelwaveguides of a plurality of channel waveguides 103 of the MUX/DEMUXs106 shown in FIGS. 28 and 29(a), respectively. Each of the channelwaveguides 131 to 133 shown in FIGS. 30(a) and 30(b) has positions (darkpoints) of a “crest” and positions (blank points) of a “hollow” of alight wave. Here, where a light wave propagating in the channelwaveguides 131 to 133 is represented by cos(α) (α represents the phase),the “crest” represents the position at which the phase a is 2×n×π andthe “hollow” represents the position at which the phase a is (2n+1)×π.It is to be noted that n and π represent a positive integer and thenumber π, respectively.

[0017] Accordingly, in each of FIGS. 31(a) and 31(b), the length betweentwo adjacent “crests” is equal to the wavelength of the light wavepropagating in the channel waveguides 131 to 133. In particular, thelight wavelengths shown in FIGS. 30(a) and 30(b) are equal to λ₀ and λ₁,respectively.

[0018]FIG. 30(a) shows a phase of light when light which has awavelength equal to a central wavelength in a light wavelengtharrangement used for wavelength division multiplex transmission. Thelength of each of the channel waveguides 103 is designed such that anaccurately integral number of waves of light of the central wavelength Aamong the wavelengths of the wavelength division multiplexed light maybe included therein. More particularly, in the case of FIG. 30(a), thelengths of the channel waveguides 103 are designed such that nine wavesof the central wavelength λ₀ are included in the shortest waveguide 131,ten waves of the central wavelength λ₀ are included in the middlewaveguide 132, and eleven waves of the central wavelength λ₀ areincluded in the longest waveguide 133.

[0019] For example, as seen in FIG. 31, when the channels #1 to #11 areset in the order from a short wavelength band, the wavelength of thelight set to the channel #6 corresponds to the central wavelength λ₀described above.

[0020] In particular, as seen in FIG. 30(a), light waves which have acomponent of a central wavelength to be outputted from the waveguides131 to 133 have the same phase at the position of a slab boundary line142 between the output slab 104 and the waveguides 131 to 133. In otherwords, an equiphase wave surface p1 of the light waves of the wavelengthA 0 outputted from the channel waveguides 103 is perpendicular to thewaveguides 131 to 133, and the lights outputted from the threewaveguides 131 to 133 are diffracted to an accurately horizontaldirection d1 with respect to the output azimuths of the waveguides 131to 133.

[0021] However, as seen in FIG. 30(b), light waves of the wavelength λ1shorter by Δλ than that of the central wavelength component do not havethe same phase at the position of the slab boundary line 142 between theoutput slab 104 and the waveguides 131 to 133, but have the same phaseat another position shifted in a unit of Δλ among the neighboringwaveguides 131 to 133. In other words, an equiphase wave surface p2 ofthe light waves of the wavelength λ1 is not perpendicular to thewaveguides 131 to 133, and also the lights outputted from the waveguides131 to 133 are diffracted to an upper side direction d2 in FIG. 30(b).

[0022] It is to be noted that light waves whose wavelength is longer byΔλ than the central wavelength λ0 are diffracted to a lower sidedirection in FIG. 30(b) in accordance with a principle similar to thatdescribed above. Accordingly, since the diffraction direction(diffraction angle) by each of the channel waveguides 103 depends uponthe value of the optical wavelength of the wavelength divisionmultiplexed light, the channel waveguides 103 can demultiplex thewavelength division multiplexed light.

[0023] The output slab 104 condenses the lights diffracted inpredetermined diffraction directions for the individual wavelengths andmultiplexed by the channel waveguides 103 and supplies the condensedlights to the output waveguides 105 of corresponding channels.

[0024] On the contrary, if lights of particular wavelengths (usually,lights of a spectrum of a width smaller than the bandwidth of theMUX/DEMUX 106 are used for WDM communication) corresponding to lights tobe outputted to the channels #1 to #n (for example, outputs of ch#1 toch#11 shown in FIG. 31) are inputted to the output waveguides 105 (referto FIG. 28) for the outputs of the channels #1 to #n, then all of thelights are multiplexed and outputted from the input waveguide 101 (referto FIG. 28).

[0025]FIG. 31 illustrates an example of the spectral characteristic andthe insertion loss of the MUX/DEMUX 106 described above with referenceto FIGS. 28 and 29(a). If wavelength division multiplexed light for 11channels (channel (ch) #1 to channel #11) is inputted to the inputwaveguide 101, then the output waveguides 105 outputs lights with suchintensities as seen from the channels #1 to #11 of FIG. 31.

[0026] A basic configuration and operation of an AWG which is anapparatus relating to the present invention are disclosed, for example,in “IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS. VOL. 2 No.2, pp.236-250 (1996)” and so forth. The wavelength division multiplexingand demultiplexing apparatus according to the present invention issimilar, in regard to the configuration, function and operation otherthan those of the characteristic part of the present invention, to thosedisclosed in the reference document mentioned above.

[0027] The insertion loss of the MUX/DEMUX 106 is a loss at which thetransmission factor for each of the channels #1 to #n of the outputwaveguides 105 exhibits a maximum value, or in other words, a loss witha wavelength with which the loss is lowest with respect to input light,and differs among different channels. For example, as seen in FIG. 31,the insertion loss of the MUX/DEMUX 106 differs among the outputchannels (#1 to #n).

[0028] Such insertion loss as described above with reference to FIG. 31occurs principally at a connection location between the input slab 102and each of the channel waveguides 103 (refer to a slab boundary line122 shown in FIG. 30; hereinafter referred to as node) and a nodebetween each of the channel waveguides 103 and the output slab 104(refer to the slab boundary line 142 shown in FIGS. 30(a) and 30(b)).

[0029] FIGS. 32(a) to 32(c) are views each illustrating a factor ofoccurrence of the insertion losses at the nodes between the input slab102 and the channel waveguides 103 described above. Particularly, FIG.32(a) shows essential part of the MUX/DEMUX 106, and FIG. 32(b) showsthe input slab 102 in an enlarged scale while FIG. 32(c) shows the nodesbetween the input slab 102 and the channel waveguides 103 in a furtherenlarged scale.

[0030] If attention is paid to the slab boundary line 122 on which thenodes between the input slab 102 and the channel waveguides 103 arepositioned as illustrated in FIG. 32(c), of light 8 advancing from theinput slab 102 toward a channel waveguide 103, light 85 which istransmitted by the channel waveguide 103 is valid, but light whicharrives at a gap portion 123 is scattered and makes invalid light 86 andtherefore becomes loss.

[0031] As a first countermeasure for reducing such insertion loss asdescribed above, it is a possible idea to use such an input slab 102-1as shown in FIG. 33(a) as the input slab for the MUX/DEMUX 106 shown inFIG. 28. The input slab 102-1 shown in FIG. 33(a) has a reduced channelwaveguide distance dc so that the connection loss of the channelwaveguides 103 is reduced.

[0032] In particular, if the channel waveguide distance dc is reduced ina condition that the width w, the focal length f (distance from thechannel waveguide center 21 to the incoming position of the channelwaveguide 103) and the channel waveguide number are fixed as illustratedin FIG. 33(b), then the connection loss of the channel waveguides can bereduced.

[0033] In the following, subjects to be solved are described inparagraphs (1-1) to (1-3).

[0034] (1-1) In the MUX/DEMUX 106 shown in FIG. 28, however, since theshape of the nodes between the input slab 102 and the channel waveguides103 and the shape of the nodes between the output slab 104 and thechannel waveguides 103 are symmetrical to each other, if the channelwaveguide distance dc (refer to FIG. 33(a)) between the channelwaveguides 103 of the input slab 102 side is decreased, then also thedistance (not shown) between the channel waveguides 103 of the outputslab 104 side decreases. In this instance, a disadvantage occurs that,in the proximity of the output slab 104 described below, lightspropagating in the channel waveguides 103 join together and interferewith each other.

[0035] In particular, an optical waveguide has a characteristic that, asa plurality of waveguides come close to each other to make the distancetherebetween small, lights propagating in the waveguides join together.Therefore, if the distance between the channel waveguides 103 is madesmall, lights propagating in the channel waveguides 103 join together inthe proximity of the output slab 104 and interfere with each other.Further, as shown in FIG. 30(b), the MUX/DEMUX 106 functions as awavelength division multiplexing and demultiplexing apparatus since aphase difference is produced among lights propagating in the channelwaveguides 131 to 133 at the output apertures 142 of the channelwaveguides 131 to 133.

[0036] Here, if it is assumed that the distance between the channelwaveguides 131 to 133 in the proximity of the output slab 104 decreasesuntil lights propagating in the channel waveguides 131 to 133 jointogether, then the phase varies and the wavelength divisiondemultiplexing function drops (depresses). Accordingly, the MUX/DEMUX106 illustrated in FIG. 28 has a subject to be solved in that it isimpossible to decrease the distance (channel waveguide distance dc shownin FIGS. 32(c) and 33(a)) between the nodes between the input slab 102and the channel waveguides 103 and the distance (not shown) between thenodes between the output slab 104 and the channel waveguides 103 asmeans for reducing the insertion losses.

[0037] (1-2) As a second countermeasure for reducing the connection(scattering) loss of the input slab 102 and output slab 104 and thechannel waveguides 103 shown in FIG. 28, it is a possible idea to form,for example, such channel waveguides 103-1 as shown in FIG. 34(a).

[0038] In particular, as seen in FIG. 34(a), at an input side node 107at which the channel waveguides 103-1 are connected to the input slab102, tapering connection branches 162 whose width reduces as thedistance from the input slab 102 increases are formed (in the followingdescription, such a pattern that the waveguide width changes smaller aswith the tapering connection branches 162 is referred to as taperingpattern or tapering).

[0039] In the countermeasure illustrated in FIG. 34(a), the scatteringloss of the input side node 107 decreases as the width with which thetapering connection branches 162 are connected to the input slab 102increases.

[0040] However, in the MUX/DEMUX 106 to which the channel waveguides103-1 having such tapering connection branches 162 as described abovewith reference to FIG. 34(a) are applied, such higher-order mode lightas hereinafter described is excited in the tapering connection branches162 formed between the input slab and the channel waveguides ashereinafter described, and the excited higher-order mode light isradiated to the outside of the channel waveguides (core), resulting inloss.

[0041] Light incoming to the tapering connection branch 162 from theinput slab 102 propagates in the tapering connection branch 162 formedas a core while the intensity peak is split into two (at a location atwhich higher-order mode light is excited) and then joined back into one.In the process wherein the number of peaks varies, part of the light(which corresponds to the higher-order mode light) is radiated to theoutside of the channel waveguide (core) 103-1, resulting in loss.

[0042] Accordingly, also the MUX/DEMUX to which the channel waveguide103-1 shown in FIG. 34(a) is applied has a subject to be solved in thatit suffers from intensity-demultiplex light loss of higher-order modelight radiated to the outside of the channel waveguide 103-1.

[0043] (1-3) As described in paragraph (1-1) above, it is necessary toprevent joining together of lights propagating in the channel waveguides103 at the output apertures of the channel waveguides 103 and keep thephase difference (phase difference between lights which propagate, forexample, in the channel waveguides 103 of FIG. 30(b)) from whichwavelength division multiplexing and demultiplexing operations arise. Tothis end, it is demanded to keep the distance between the channelwaveguides 103 of the output slab 104 side great.

[0044] Further, it is necessary to make the gap (for example, the gapportion 123 shown in FIG. 32(c)) small. To this end, it is demanded tomake the distance between the channel waveguides 103 of the input slab102 side (channel waveguide distance dc shown in FIGS. 32(c) and 33(a))small.

SUMMARY OF THE INVENTION

[0045] It is an object of the present invention to provide a wavelengthdivision multiplexing and demultiplexing apparatus of the type whereinthe shapes of an input slab and an output slab are symmetrical to eachother, by which, while the distance between channel waveguides at nodesbetween the output slab and the channel waveguides is kept great, thedistance between the channel waveguides at nodes between the input slaband the channel waveguides can be made small thereby to reduce the loss.

[0046] It is another object of the present invention to provide awavelength division demultiplexing apparatus which can suppressexcitation of higher-order mode light to reduce the loss caused by suchhigher-order mode light.

[0047] In order to attain the object described above, according to anaspect of the present invention, there is provided a wavelength divisiondemultiplexing apparatus, comprising a substrate, a first waveguide,disposed on the substrate, for propagating wavelength divisionmultiplexed light having a plurality of wavelength components, a firstslab, disposed on the substrate, for diffusing the wavelength divisionmultiplexed light inputted from the first waveguide, a plurality ofchannel waveguides, disposed on the substrate and having a series ofdifferent waveguide lengths increasing with a predetermined difference,for receiving and propagating the wavelength division multiplexed lightdiffused by the first slab, separately from each other channelwaveguide, a second slab, disposed on the substrate, for receiving thewavelength division multiplexed light separately propagated through theplural channel waveguides and demultiplexing the received wavelengthdivision multiplexed light into the plurality of wavelength componentswith condensing each of the plural wavelength components, and a secondwaveguide, disposed on the substrate, for propagating the lightdemultiplexed by the second slab therein, the channel waveguides and thefirst slab being optically connected to each other at a number of nodesgreater than the number of nodes at which the channel waveguides and thesecond slab are connected to each other.

[0048] According to another aspect of the present invention, there isprovided a wavelength division demultiplexing apparatus, comprising asubstrate, a first waveguide, disposed on the substrate, for propagatingwavelength division multiplexed light having a plurality of wavelengthcomponents, a first slab, disposed on the substrate, for diffusing thewavelength division multiplexed light inputted from the first waveguide,a plurality of channel waveguides, disposed on the substrate and havinga series of different waveguide lengths increasing with a predetermineddifference, for receiving and propagating the wavelength divisionmultiplexed light diffused in the first slab, separately from each otherchannel waveguide, a second slab, disposed on the substrate, forreceiving the wavelength division multiplexed light separatelypropagated through the plural channel waveguides and demultiplexing thereceived wavelength division multiplexed light into the plurality ofwavelength components with condensing each of the plural wavelengthcomponents, and a second waveguide, disposed on the substrate, forpropagating the light demultiplexed by the second slab therein, each ofthe channel waveguides having, in the proximity of a portion thereof atwhich the channel waveguide is optically connected to the first slab, aplurality of branches or waveguides through core to which the wavelengthdivision multiplexed light from the first slab is inputted and a mergingportion formed integrally with the branches or waveguides through corefor optically coupling the wavelength division multiplexed light fromthe branches or waveguides through core.

[0049] In this instance, preferably each of the branches or waveguidesthrough core has a width with which higher-order mode light of thewavelength division multiplexed light inputted thereto is cut off, thehigher-order mode light being light of a mode or modes higher than thezero order mode, and a coupling contact at the merging portion is formedwith a width with which the higher-order mode light of the distributedlight inputted thereto can be excited.

[0050] Further preferably, each of the branches or waveguides throughcore is formed with a width which decreases in a tapering fashion from aportion thereof adjacent the merging portion toward the first slab.

[0051] Each of the branches or waveguides through core may have atapering portion having a width which decreases in a tapering fashionfrom a portion thereof adjacent the merging portion toward the firstslab and a fixed small width waveguide having a substantially fixedwidth substantially equal to the width of the tapering portion at aposition at which the tapering portion has the smallest width foroptically connecting the first slab and the tapering portion to eachother.

[0052] In this instance, a boundary interface of the first slab to eachof the channel waveguides may be formed in an arc centered at the centerof diffusion of the light diffused in and inputted from the first slabto the boundary interface, and each of the branches or waveguidesthrough core may have a center axis disposed on an extension line fromthe center of diffusion.

[0053] According to a further aspect of the present invention, there isprovided a wavelength division demultiplexing apparatus, comprising asubstrate, a first waveguide, disposed on the substrate, for propagatingwavelength division multiplexed light having a plurality of wavelengthcomponents, a first slab, disposed on the substrate, for diffusing thewavelength division multiplexed light inputted from the first waveguide,a plurality of channel waveguides, disposed on the substrate and havinga series of different waveguide lengths increasing with a predetermineddifference, for receiving and propagating the wavelength divisionmultiplexed light diffused in the first slab, separately from each otherchannel waveguide, a second slab, disposed on the substrate, forreceiving the wavelength division multiplexed light separatelypropagated through the plural channel waveguides and demultiplexing thereceived wavelength division multiplexed light into the plurality ofwavelength components with condensing each of the plural wavelengthcomponents, and a second waveguide, disposed on the substrate, forpropagating the light demultiplexed by the second slab therein, each ofthe channel waveguides having, in the proximity of a portion thereof atwhich the channel waveguide is optically connected to the first slab, aplurality of sets of primary coupling portions each including aplurality of primary branching connection branches for receiving thewavelength division multiplexed light from the first slab and a primarymerging portion for optically coupling the wavelength divisionmultiplexed light from the primary branching connection branches, and asecondary coupling portion including a plurality of secondary branchingconnection branches for receiving the wavelength division multiplexedlight coupled by the primary coupling portions and a secondary mergingportion for optically coupling the wavelength division multiplexed lightfrom the secondary branching connection branches.

[0054] In this instance, preferably each of the branching connectionbranches has a width with which higher-order mode light of thewavelength division multiplexed light inputted thereto is cut off, and acoupling contact at the merging portion is formed with a width withwhich the higher-order mode light of the wavelength division multiplexedlight inputted thereto can be excited.

[0055] Further, a boundary interface of the first slab to each of thechannel waveguides may be formed in an arc centered at the center ofdiffusion of the light diffused in and inputted from the first slab tothe boundary interface, and each of the channel waveguides in theproximity of a portion at which the channel waveguide is opticallyconnected to the first slab may have a center axis disposed on anextension line from the center of diffusion. Meanwhile, each of thebranching connection branches may be formed with a width which decreasesin a tapering fashion from a portion thereof adjacent the mergingportion toward the first slab. Preferably, each of the branchingconnection branches has a tapering portion having a width whichdecreases in a tapering fashion from a portion thereof adjacent themerging portion toward the first slab and a fixed small width waveguidehaving a substantially fixed width substantially equal to the width ofthe tapering portion at a position at which the tapering portion has thesmallest width for optically connecting the first slab and the taperingportion to each other.

[0056] According a still further aspect of the present invention, thereis provided a wavelength division demultiplexing apparatus, comprising asubstrate, a first waveguide, disposed on the substrate, for propagatingwavelength division multiplexed light having a plurality of wavelengthcomponents, a first slab, disposed on the substrate, for diffusing thewavelength division multiplexed light inputted from the first waveguide,a plurality of channel waveguides, disposed on the substrate and havinga series of different waveguide lengths increasing with a predetermineddifference, for receiving and propagating the wavelength divisionmultiplexed light diffused in the first slab, separately from each otherchannel waveguide, a second slab, disposed on the substrate, forreceiving the wavelength division multiplexed light separatelypropagated through the plural channel waveguides and demultiplexing thereceived wavelength division multiplexed light into the plurality ofwavelength components with condensing each of the plural wavelengthcomponents, and a second waveguide, disposed on the substrate, forpropagating the light demultiplexed by the second slab therein, each ofthe channel waveguides being formed such that a node thereof to thefirst slab has a width with which higher-order mode light of theseparated light can be excited and the width thereof decreases in atapering fashion away from the first slab, an island-shaped formationregion of a reflection index lower than that of the channel waveguidesbeing provided for each of the channel waveguides in such a manner as topartition the channel waveguide in the proximity thereof at which thechannel waveguide is optically connected to the first slab into aplurality of waveguide portions.

[0057] In this instance, each of the waveguide portions of each of thechannel waveguides partitioned by the island-shaped region may be formedas a waveguide by which higher-order mode light of the wavelengthdivision multiplexed light inputted thereto is cut off, and thewaveguide width at a portion at which the partitioned waveguide portionsare coupled to each other may have a width with which the higher-ordermode light of the distributed light inputted thereto can be excited.

[0058] Further, in the wavelength division demultiplexing apparatus, aboundary interface of the first slab to each of the channel waveguidesmay formed in an arc centered at the center of diffusion of the lightdiffused in and inputted from the first slab to the boundary interface,and further, each of the channel waveguides in the proximity of aportion at which the channel waveguide is optically connected to thefirst slab may have a center axis disposed on an extension line from thecenter of diffusion.

[0059] According to a yet further aspect of the present invention, thereis provided a wavelength division demultiplexing apparatus, comprising asubstrate, a first waveguide, disposed on the substrate, for propagatingwavelength division multiplexed light having a plurality of wavelengthcomponents, a first slab, disposed on the substrate, for diffusing thewavelength division multiplexed light inputted from the first waveguide,a plurality of channel waveguides, disposed on the substrate and havinga series of different waveguide lengths increasing with a predetermineddifference, for receiving and propagating the wavelength divisionmultiplexed light diffused in the first slab, separately from each otherchannel waveguide, a second slab, disposed on the substrate, forreceiving the wavelength division multiplexed light separatelypropagated through the plural channel waveguides and demultiplexing thereceived wavelength division multiplexed light into the plurality ofwavelength components with condensing each of the plural wavelengthcomponents, and a second waveguide, disposed on the substrate, forpropagating the light demultiplexed by the second slab therein, each ofthe channel waveguides including, in the proximity of a portion thereofat which the channel waveguide is optically connected to the first slab,a plurality of coupling waveguides connected in tandem in a plurality ofstages in a tree-like configuration for optically coupling andpropagating the distributed light inputted thereto.

[0060] The above and other objects, features and advantages of thepresent invention will become apparent from the following descriptionand the appended claims, taken in conjunction with the accompanyingdrawings in which like parts or elements denoted by like referencesymbols.

BRIEF DESCRIPTION OF THE DRAWINGS

[0061]FIG. 1 is a schematic view showing a wavelength divisionmultiplexing and demultiplexing apparatus which functions as awavelength division demultiplexing apparatus according to a firstembodiment of the present invention;

[0062]FIGS. 2 and 3 are schematic views showing part of the wavelengthdivision multiplexing and demultiplexing apparatus of FIG. 1;

[0063]FIG. 4(a) is a schematic view illustrating propagation of light ina channel waveguide of a conventional wavelength division multiplexingand demultiplexing apparatus and FIG. 4(b) is a similar view butillustrating operation of the wavelength division multiplexing anddemultiplexing apparatus of FIG. 1;

[0064]FIG. 5is a schematic view showing part of a wavelength divisionmultiplexing and demultiplexing apparatus according to a modification tothe wavelength division multiplexing and demultiplexing apparatus ofFIG. 1;

[0065]FIG. 6 is a schematic view showing a wavelength divisionmultiplexing and demultiplexing apparatus which functions as awavelength division demultiplexing apparatus according to a secondembodiment of the present invention;

[0066]FIG. 7 is a schematic view showing part of the wavelength divisionmultiplexing and demultiplexing apparatus of FIG. 6;

[0067]FIG. 8(a) is a schematic view illustrating propagation of light ina channel waveguide of a conventional wavelength division multiplexingand demultiplexing apparatus and FIG. 8(b) is a similar view butillustrating operation of the wavelength division multiplexing anddemultiplexing apparatus of FIG. 6;

[0068]FIG. 9 is a schematic view showing a wavelength divisionmultiplexing and demultiplexing apparatus which functions as awavelength division demultiplexing apparatus according to a thirdembodiment of the present invention;

[0069]FIG. 10 is a schematic view showing part of the wavelengthdivision multiplexing and demultiplexing apparatus of FIG. 9;

[0070]FIG. 11 is a schematic view showing part of a wavelength divisionmultiplexing and demultiplexing apparatus according to a modification tothe wavelength division multiplexing and demultiplexing apparatus ofFIG. 9;

[0071]FIG. 12 is a schematic view showing a wavelength divisionmultiplexing and demultiplexing apparatus which functions as awavelength division demultiplexing apparatus according to a fourthembodiment of the present invention;

[0072]FIGS. 13 and 14 are schematic views showing part of the wavelengthdivision multiplexing and demultiplexing apparatus of FIG. 12;

[0073]FIG. 15(a) is a schematic view illustrating operation of an inputside node of the wavelength division multiplexing and demultiplexingapparatus of FIG. 12 and FIG. 15(b) is a similar view but illustratingoperation of an input side node of the wavelength division multiplexingand demultiplexing apparatus of FIG. 1;

[0074]FIG. 16 is a schematic view showing part of a wavelength divisionmultiplexing and demultiplexing apparatus according to a modification tothe wavelength division multiplexing and demultiplexing apparatus ofFIG. 12;

[0075]FIG. 17 is a schematic view illustrating propagation of light in achannel waveguide of a conventional wavelength division multiplexing anddemultiplexing apparatus;

[0076]FIG. 18 is a schematic view illustrating operation of the modifiedwavelength division multiplexing and demultiplexing apparatus of FIG.16;

[0077]FIG. 19 is a schematic view showing a wavelength divisionmultiplexing and demultiplexing apparatus which functions as awavelength division demultiplexing apparatus according to a fifthembodiment of the present invention;

[0078]FIGS. 20 and 21 are schematic views showing part of the wavelengthdivision multiplexing and demultiplexing apparatus of FIG. 19;

[0079]FIG. 22(a) is a schematic view illustrating operation of thewavelength division multiplexing and demultiplexing apparatus of FIG. 19and FIG. 22(b) is a schematic view illustrating propagation of light ina channel waveguide of the wavelength division multiplexing anddemultiplexing apparatus shown in FIGS. 12 and 13;

[0080]FIG. 23 is a schematic view showing part of a wavelength divisionmultiplexing and demultiplexing apparatus according to a modification tothe wavelength division multiplexing and demultiplexing apparatus ofFIG. 19;

[0081] FIGS. 24 to 27 are schematic views showing several wavelengthdivision multiplexing and demultiplexing apparatus which each functionsas a wavelength division multiplexing and demultiplexing apparatusaccording to different embodiments of the present invention;

[0082]FIG. 28 is a schematic view showing an ordinary wavelengthdivision multiplexing and demultiplexing apparatus;

[0083]FIG. 29(a) is a schematic view showing a configuration ofwaveguides of a wavelength division multiplexing and demultiplexingapparatus of the AWG type, FIG. 29(b) is a view showing an example of aconfiguration of a conventional spectroscope, and FIG. 29(c) is a viewillustrating a corresponding relationship between components of awavelength division multiplexing and demultiplexing apparatus configuredusing a waveguide and a conventional spectroscope;

[0084] FIGS. 30(a) and 30(b) are schematic views illustrating aprinciple of operation of a waveguide type diffraction grating;

[0085]FIG. 31 is a diagrammatic view illustrating operation of thewaveguide type diffraction grating shown in FIGS. 30(a) and 30(b);

[0086] FIGS. 32(a) to 32(c) are schematic views illustrating a factor ofoccurrence of insertion loss at a node between an input slab and channelwaveguides;

[0087]FIG. 33(a) is a schematic view illustrating a first countermeasurefor reducing the insertion loss, and FIG. 33(b) is a diagramillustrating operation of the first countermeasure shown in FIG. 33(a);

[0088]FIG. 34(a) is a schematic view illustrating a secondcountermeasure for reducing the insertion loss, and FIG. 34(b) is adiagram illustrating operation of the second countermeasure shown inFIG. 34(a); and

[0089]FIG. 35 is a schematic view showing an example of a configurationof a conventional spectroscope.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0090] (a) First Embodiment FIG. 1 shows principal components of aMUX/DEMUX 10 to which a wavelength division demultiplexing apparatusaccording to a first embodiment of the present invention is applied andparticularly shows a pattern of a core of optical waveguide deviceswhich compose the MUX/DEMUX 10. Further, FIG. 2 particularly shows acore pattern of connection portions between an input slab 2 and channelwaveguides 3 which are components of the MUX/DEMUX 10.

[0091] The MUX/DEMUX 10 according to the first embodiment is formedfrom, for example, an under clad layer having a refractive index ofapproximately 1.551 and a thickness of approximately 20 μm, a corehaving a refractive index of approximately 1.5588 and a thickness ofapproximately 7 μm and an over clad layer having a refractive index ofapproximately 1.551 and a thickness of approximately 20 μm, all formedon, for example, such a silicon substrate 100 as described hereinabovewith reference to FIG. 28 by a combination of deposition of SiO₂ by aCVD (Chemical Vapor Deposition) method and a photolithography process.

[0092] In particular, the core of the MUX/DEMUX 10 described above isformed such that upper, lower and left, right portions thereof aresurrounded by the under clad or the over clad. Consequently, the core iscovered with the clad layers having a comparatively low refractive indexso that light can be propagated in a confined state along the core.

[0093] As seen in FIG. 1, the core of the MUX/DEMUX 10 has a patternformed integrally thereon which functions as an input waveguide (firstwaveguide) 1 for propagating wavelength division multiplexed light of aplurality of channels, an input slab (first slab) 2 for diffusing thelight inputted from the input waveguide 1, a plurality of channelwaveguides 3 having a series of different waveguide lengths successivelyincreasing with a predetermined difference for receiving and propagatingthe wavelength division multiplexed light diffused by the input slab 2separately from each other channel waveguide, an output slab (secondslab) 4 for receiving the wavelength division multiplexed lightseparately propagated through the plural channel waveguides 3 anddemultiplexing the received wavelength division multiplexed light intothe plurality of wavelength components with condensing each of theplural wavelength components, and output waveguides 5 for propagatingthe light condensed by the output slab 4 therein.

[0094] The individual components are described in more detail.

[0095] The input waveguide 1 guides light inputted thereto from the leftside in FIG. 1 and delivers the light to the input slab 2. Since theinput slab 2 has such a shape that it expands in a direction parallel toa substrate (for example, an element denoted by reference numeral 100 inFIG. 28), the light propagating in the input slab 2 is not confined butdiffuses (diverges) in a lateral direction. Therefore, the light comingto the input slab 2 through the input waveguide 1 diffuses radially fromthe center at an incoming light diffusion center 21 and comes to thechannel waveguides 3. Here, the shape of a slab boundary line 22 (referto FIG. 2) between the input slab 2 and the channel waveguides 3 is anarc of a radius f centered at the incoming light diffusion center 21.Therefore, the light diffused from the incoming light diffusion center21 shown in FIG. 1 is introduced with the same phase into the pluralityof channel waveguides 3. The channel waveguides 3 are formed such thatthe differences between lengths of adjacent ones thereof from the inputslab 2 to the output slab 4 are equal to each other.

[0096] Due to the differences in length, when lights introduced intoadjacent ones of the channel waveguides 3 pass the channel waveguides 3and come to output apertures 44 of the channel waveguides 3, theyexhibit such phase differences as seen in FIG. 30(b). Here, the outputapertures 44 correspond to the connection portions 44 between thechannel waveguides 3 and the output slab 4. Since the channel waveguides3 have a function of generating such phase differences, they are calledphased array. Further, the difference in length between adjacent ones ofthe channel waveguides 3 is designed equal to m times the centerwavelength λ0. Here, m is a positive integer and is called order ofchannel waveguides 3 or order of the phased array.

[0097] Then, the lights passing the channel waveguides 3 and coming tothe output apertures 44 of the channel waveguides 3 have an equiphasewave surface which is different depending upon the wavelength. Theequiphase wave surface is such as that, for example, denoted by d1 or d2shown in FIG. 30(a) or 30(b).

[0098] Further, in the output slab 4, a slab boundary line 42 betweenthe channel waveguides 3 and the output slab 4 and the output apertures44 are both formed in a curve of the radius r similarly as in the inputslab 2. Therefore, the lights outputted from the output apertures 44 ofthe channel waveguides 3 to the output slab 4 are condensed at thecenter of the arc of the radius r which defines the arrangementpositions of the output slab 4 and the output apertures 44. Strictly,where the wavelength of the lights outputted from the output apertures44 of the channel waveguides 3 is shorter than the central wavelength,the lights are condensed on the relatively upper side in FIG. 2, butwhere the wavelength of the lights outputted from the output apertures44 of the channel waveguides 3 is longer than the central wavelength,the lights are condensed on the relatively lower side in FIG. 1.

[0099] Furthermore, the output waveguide 5 is disposed such that one endthereof is positioned at a position at which lights of a desiredwavelength are condensed, and the other end of the output waveguide 5 isused as an output terminal. Usually, an optical fiber or an inputterminal of some other optical part is connected to the output end ofthe output waveguide 5.

[0100] For the input waveguide 1, input slab 2, output slab 4 and outputwaveguides 5, elements basically similar to those described hereinabovewith reference to FIG. 28 (refer to reference numerals 101, 102, 104 and105) can be used.

[0101] As described hereinabove, the channel waveguides 3 have a seriesof different waveguide lengths such that each adjacent ones of thechannel waveguides have a predetermined waveguide length differencetherebetween so that light propagating in each of the channel waveguides3 is deflected (demultiplexed) to predetermined particular angles whichdiffer among different wavelengths of the wavelength divisionmultiplexed light and the thus deflected lights are outputted to theoutput slab 4. Further, intermediate portions of the channel waveguides3 are formed in a spaced relationship by a necessary distance from eachother so that the lights propagating therein may not interfere with eachother.

[0102] It is to be noted that the width of the input waveguide land theoutput waveguides 5 (the width of the core pattern) and the waveguidewidth of the intermediate portions of the channel waveguides 3 exceptthe opposite end portions 6 and 7 (the width of the core pattern) can beset to approximately 7 μm.

[0103] Further, by setting the central wavelength λ₀ to 1.552 μm, theorder number m of the channel waveguides 3 to 30 and the effectivereflection index of the channel waveguides 3 to approximately 1.552, thedifference in length between adjacent ones of the channel waveguides canbe set to approximately 30 μm.

[0104] Meanwhile, the end portions 7 of the channel waveguides 0.3adjacent the output slab 4 are formed in such a shape that the widththereof decreases in a tapering fashion as the distance from the outputslab 4 increases. In particular, the distance dc2 between the channelwaveguides 3 at the nodes thereof to the output slab 4 is 22 μm whilethe width at the end of the taper (Wmax in FIG. 1) is 19 μm, and thelength of the tapering portions of the channel waveguides 3 is 2.5 mm.

[0105] The radii f of the input slab 2 and the output slab 4 of theMUX/DEMUX 10 in the first embodiment are both approximately 6.2 mm, andthe widths of the input slab 2 and the output slab 4 are approximately 1mm. In other words, the slab boundary line 22, nodes 24, slab boundaryline 42 and connection portions 44 are all disposed on an arc of theradius of 6.2 mm.

[0106] Here, the end portions 6 of the channel waveguides 3 adjacent theinput slab 2 have such a configuration as described below, which is acharacteristic of the present invention.

[0107] In particular, the number of nodes at which each of the channelwaveguides 3 and the output slab 4 are optically connected to each other(that is, the number of nodes 24 between the input slab 2 and each ofthe channel waveguides 3) is greater than the number of connectionportions at which each of the channel waveguides 3 and the input slab 2are connected to each other. More particularly, as seen in FIG. 3, aneighboring portion of each of the channel waveguides 3 to a portion atwhich the channel waveguide 3 is optically connected to the input slab2, that is, the end portion 6 of each of the channel waveguides 3adjacent the input slab 2, is formed from two branching connectionbranches 61 to which wavelength division multiplexed light from theinput slab 2 is inputted and a merging portion 69 formed integrally withthe branching connection branches 61 for optically coupling thewavelength division multiplexed light from the branching connectionbranches 61.

[0108] Consequently, the distance between the locations at which thechannel waveguides 3 and the input slab 2 are optically connected toeach other (for example, a channel waveguide distance dc11 or dc12 shownin FIG. 2) can be reduced, and the amount of loss caused by the gapportions (such portions as that denoted by reference numeral 123 in FIG.32(c)) is reduced. It is to be noted that, in this instance, the channelwaveguide distance (dc1 in FIG. 2) at the nodes 24 between the inputslab 2 and the channel waveguides 3 can be set typically toapproximately 22 μm, the distance (dc11 in FIGS. 1 and 2) between thebranching connection branches 61 on the slab boundary line 22 toapproximately 11 μm, the waveguide length from the slab boundary line 22to the location at which the branching connection branches 61 jointogether at the merging portion 69 to approximately 5 mm, and thewaveguide length of the tapering portion after the branching connectionbranches 61 join together to approximately 1 mm.

[0109] The two branching connection branches 61 shown in FIGS. 2 and 3are formed such that a center axis 31 of the end portion 6 of each ofthe channel waveguides 3 adjacent the input slab 2 passes the center ofa gap portion G1 positioned between the two branching connectionbranches 61 and an extension line of the center axis 31 passes theincoming light diffusion center 21. In other words, the center axis 31of the end portion 6 of each of the channel waveguides 3 coincides withthe optical axis of incoming light to the channel waveguide 3.

[0110] Further, each of the two branching connection branches 61 isconfigured such that it has a width W2 with which higher-order modelight of wavelength division multiplexed light inputted thereto from theinput slab 2 is cut off. Further, a coupling point of the mergingportion 69 between the two branching connection branches 61 is formedsuch that it has a width W1 with which higher-order mode light ofwavelength division multiplexed light inputted thereto is excited.

[0111] An effect provided by the formation just described is describedthrough comparison in operation between the MUX/DEMUX 10 of the presentinvention and a conventional MUX/DEMUX with reference to FIGS. 4(a) and4(b).

[0112]FIG. 4(a) illustrates propagation of light in a channel waveguideof a conventional MUX/DEMUX. Referring to FIG. 4(a), if incoming light 8is introduced into each of the tapering connection branches 162, thenwhile almost all of the light propagates as 0th-order mode light 80 a,part of the light is excited as second-order mode light 82 in theproximity of the node 24 between the channel waveguide 3 and the inputslab 2. However, since the channel waveguide 162 tapers toward the rightside in FIG. 4(a), the second-order mode light 82 is cut off after itadvances by a predetermined distance in the channel waveguide 162. As aresult, the second-order mode light 82 is radiated to the outside of thewaveguide (core) 162 (refer to reference character P22 in FIG. 4(a)) andbecomes loss.

[0113] In contrast, in each of the channel waveguides 3 shown in FIGS. 1to 3, each of the branching connection branches 61 is formed from awaveguide of the small width W2 so that higher-order mode light may becut off there.

[0114]FIG. 4(b) illustrates operation of the MUX/DEMUX in the firstembodiment. If incoming light 8 is introduced into each of the branchingconnection branches 61 as seen in FIG. 4(b), then only the 0th-ordermode light 80 a propagates along the branching connection branch 61, andtherefore, no loss occurs.

[0115] Further, while the merging portion 69 is formed with the width W1with which higher-order mode light such as first-order mode light isexcited, since a first-order mode light incoming from one of thebranching connection branches 61 (for example, an upper one of thebranching connection branches 61 in FIG. 4(b)) and another first-ordermode light incoming from the other one of the branching connectionbranches 61 (for example, a lower one of the branching connectionbranches 61 in FIG. 4(b)) cancel each other, no loss by higher-ordermode light occurs.

[0116] It is to be noted that, in this instance, the waveguide length ofthe branching connection branches 61 can be set to approximately 5 mm,the waveguide length of the tapering portion after the branchingconnection branches 61 join together at the merging portion 69 toapproximately 1 mm, and the maximum width W1 of the tapering waveguideportion at the merging portion 69 to approximately 16 μm.

[0117] In the wavelength division multiplexing and demultiplexingapparatus 10 according to the first embodiment having the configurationdescribed above, if light (wavelength division multiplexed light)including a plurality of wavelength components is inputted to the inputwaveguide 1, then the wavelength division multiplexing anddemultiplexing apparatus 10 functions as a wavelength divisiondemultiplexing apparatus which outputs, at the channels #1 to #n of theoutput waveguides 5, light wavelength-demultiplexed (wavelength-split)for the individual channels. On the other hand, the wavelength divisionmultiplexing and demultiplexing apparatus 10 functions also as awavelength division demultiplexing apparatus whichwavelength-multiplexes light of the channels #1 to #n inputted to theoutput waveguides 5 thereof and outputs the wavelength divisionmultiplexed light through the input waveguide 1.

[0118] Further, the channel waveguides 3 output wavelength divisionmultiplexed light at emerging angles different among differentwavelengths to the output slab 4 similarly as in the case describedhereinabove with reference to FIG. 30 thereby to demultiplex thewavelength-multiplex light into lights of different wavelengths. Theoutput slab 4 collimates the demultiplexed lights of the differentwavelengths so that lights of the same length are condensed at theincoming point of each of the output waveguides 5. Consequently, theoutput waveguides 5 can propagate the lights having differentwavelengths among different channels from one another.

[0119] In this manner, with the wavelength division demultiplexingapparatus according to the first embodiment of the present invention,since the distances dc11 and dc12 between the nodes at which the channelwaveguides 3 and the input slab 2 are optically coupled to each otherare reduced, or in other words, since the angle pitch of wavelengthdivision multiplexed lights inputted to the end portions 6 of thechannel waveguides 3 adjacent the input slab 2 is reduced, theconnection loss between the input slab 2 and the channel waveguides 3can be reduced.

[0120] Further, with the wavelength division demultiplexing apparatusaccording to the present embodiment, such loss as is caused by radiationof higher-order mode light in a conventional wavelength divisiondemultiplexing apparatus does not occur, and therefore, the loss by thewavelength division demultiplexing apparatus is reduced.

[0121] It is to be noted that each of the branching connection branches61 may otherwise be configured such that the center axis 32 a thereof isdisposed on an extension line from the incoming light diffusion center21 as shown in FIG. 5. This configuration further reduces the loss.

[0122] While, in the first embodiment described above, each of thechannel waveguides 3 has two branching connection branches 61 and amerging portion 69, the present invention is not limited to thisconfiguration, and the wavelength division demultiplexing apparatus mayotherwise include both of a channel waveguide or waveguides having twobranching connection branches 61 and a merging portion 69 and a channelwaveguide or waveguides having no such branching geometry as thebranching connection branches 61 and the merging portion 69. Also in theconfiguration just described, at least the connection loss between theinput slab 2 and the channel waveguides 3 can be reduced.

[0123] (b) Second Embodiment

[0124]FIG. 6 shows principal components of a MUX/DEMUX 10-1 whichfunctions as a wavelength division demultiplexing apparatus according toa second embodiment of the present invention and particularly shows apattern of a core of optical waveguide devices which compose theMUX/DEMUX 10-1.

[0125] Also in the MUX/DEMUX 10-1 according to the second embodiment,similarly as in the MUX/DEMUX 10 of the first embodiment describedhereinabove, a core is formed such that upper, lower and left, rightportions thereof are surrounded by an under clad or an over clad so thatlight can be propagated in a confined state in the core.

[0126] While the MUX/DEMUX 10-1 according to the second embodiment isdifferent in configuration of channel waveguides 3-1 thereof from thatin the first embodiment (refer to reference numeral 10) describedhereinabove, the remaining configuration thereof is similar to that inthe first embodiment described above. In particular, the core has apattern formed integrally thereon which functions as an input waveguide1, an input slab 2, an output slab 4, and output waveguides 5 similar tothose of the first embodiment described hereinabove in addition to thechannel waveguides 3-1 which have a characteristic unique to the secondembodiment.

[0127] In the second embodiment, each of the channel waveguides 3-1 isconfigured such that a neighboring portion thereof to a portion at whichit is optically connected to the input slab 2, that is, an end portion6-1 of each of the channel waveguides 3-1 adjacent the input slab 2, isformed as a tapering connection branch 62 a (refer to FIG. 7) which hasa width which is maximum at a connection portion of the end portion 6-1to the input slab 2 and decreases in a tapering fashion as the distancefrom the input slab 2 increases. The connection portion of the channelwaveguide 3-1 to the input slab 2 is formed such that it has a widthWmax greater than a minimum width with which higher-order mode light ofwavelength division multiplexed light inputted to the connection portionis excited.

[0128] Further, in the proximity of the position of each of the channelwaveguides 3-1 at which the channel waveguide 3-1 has the width withwhich higher-order mode light is excited, an island-shaped formationregion 34 (refer to FIG. 7) surrounded by the area in which the channelwaveguide 3-1 is formed and having a refractive index lower than that ofthe channel waveguide 3-1 is provided.

[0129] For example, as shown in FIG. 7, also the end portion 7 of eachof the channel waveguides 3-1 is formed such that, similarly as with theend portion 6-1, the width thereof is set to the width Wmax at theconnection portion thereof to the input slab 2 and decreases in atapering fashion as the distance from the output slab 4 increases.Further, the island-shaped formation region 34 is formed at the endportion 6-1 of the channel waveguide 3-1 in such an island shape that itis surrounded by the area in which the channel waveguide 3-1 is formedto extend from a waveguide position C1 at which the channel waveguide3-1 has an approximately minimum width with which higher-order modelight is excited to a position C2 on the slab boundary line 22.

[0130] As regards the width W1 of the channel waveguide 3-1 at thewaveguide position C1 of the apex portion A of the island-shapedformation region 34, the channel waveguide 3-1 can be formed in atapering fashion approximately with at least a width (for example,approximately 16 μm) with which higher-order mode light is excited suchthat the width of an intermediate portion thereof other than the endportions 6-1 and 7 is approximately 7 μm and the tapering connectionbranch 62 a has a length of approximately 1.5 to 5 mm.

[0131] Further, the waveguides 61 a-1 and 61 a-2 partitioned by theisland-shaped formation region 34 are formed as waveguides having awidth with which higher-order mode (second-order mode) light of inputtedwavelength division multiplexed light is cut off. Further, while thewaveguides 61 a-1 and 61 a-2 join together at the waveguide position C1,the channel waveguide 3-1 at the joining position of the waveguides 61a-1 and 61 a-2 has a waveguide width with which higher-order mode lightof wavelength division multiplexed light inputted thereto is excited.

[0132] With the wavelength division demultiplexing apparatus of thesecond embodiment, since the distance between the nodes 24 at which thechannel waveguides 3 and the input slab 2 are optically connected toeach other is reduced, the amount of loss caused by the gap portions(such portions as that denoted by reference numeral 123 in FIG. 32(c))is reduced. Further, similarly as in the case of the first embodimentdescribed hereinabove, higher-order mode light is cut off by the twowaveguides 61 a-1 and 61 a-2. Further, higher-order mode (first-ordermode) light excited at the waveguide position C1 with incoming linesfrom the two waveguides 61 a-1 and 61 a-2 can cancel each other at thewaveguide position C1, and therefore, loss by radiation of higher-ordermode light is not generated, resulting in reduction of the loss.

[0133] In the channel waveguides 3-1 shown in FIGS. 6 and 7, since thewaveguides 61 a-1 and 61 a-2 as branching connection branches are formedfrom a waveguide having the reduced width W2 so that higher-order modelight may be cut off, when incoming light 8 is introduced into thewaveguides 61 a-1 and 61 a-2 as seen in FIG. 8(b), no loss occurs in thewaveguides 61 a-1 and 61 a-2 because only 0th-order mode light 80 apropagates in them.

[0134] It is to be noted that FIG. 8(a) illustrates propagation of lightin a channel waveguide of a conventional MUX/DEMUX.

[0135] Also in the MUX/DEMUX 10-1 which functions as a wavelengthdivision demultiplexing apparatus according to the second embodiment ofthe present invention having the configuration described above, whenlight (wavelength division multiplexed light) including a plurality ofwavelength components is inputted to the input waveguide 1, lightswavelength-demultiplexed (wavelength-split) for the individual channelsare outputted from the channels #1 to #n of the output waveguides 5.

[0136] Further, since the angle pitch of wavelength division multiplexedlight incoming to the end portions 6-1 of the channel waveguides 3adjacent the input slab 2 (that is, the distance between angles at whichthe wavelength division multiplexed lights come into the channelwaveguides 3) is reduced by the island-shaped formation regions 34,while a waveguide width with which higher-order mode light of thewavelength division multiplexed light is cut off is achieved, a cause ofloss such as a gap portion [refer to reference numeral 123 of FIG.32(c)] is reduced.

[0137] In particular, wavelength division multiplexed light incoming tothe end portion 6-1 of each of the channel waveguides 3 adjacent theinput slab 2 propagates in the channel waveguide 3 with higher-ordermode light thereof cut off. On the other hand, since first-order modelights cancel each other at the portion C1 at which the waveguides 61a-1 and 61 a-2 join together, only 0th-order mode light propagates inthe channel waveguide 3 and the loss of the wavelength divisionmultiplexed light is reduced.

[0138] In this manner, with the wavelength division demultiplexingapparatus according to the second embodiment of the present invention,since the distances between the nodes at which the channel waveguides3-1 and the input slab 2 are optically coupled to each other arereduced, or in other words, since the angle pitch of wavelength divisionmultiplexed lights inputted to the end portions 6-1 of the channelwaveguides 3-1 adjacent the input slab 2 is reduced, the connection lossbetween the input slab 2 and the channel waveguides 3-1 can be reduced.

[0139] (c) Third Embodiment

[0140]FIG. 9 shows principal components of a MUX/DEMUX 10-2 whichfunctions as a wavelength division demultiplexing apparatus according toa third embodiment of the present invention and particularly shows apattern of a core of optical waveguide devices which compose theMUX/DEMUX 10-2.

[0141] Also in the MUX/DEMUX 10-2 according to the third embodiment,similarly as in the embodiments described hereinabove, a core is formedsuch that upper, lower and left, right portions thereof are surroundedby an under clad or an over clad so that light can be propagated in aconfined state in the core.

[0142] While the MUX/DEMUX 10-2 according to the third embodiment isdifferent in configuration of channel waveguides 3-2 thereof from thatin the embodiments (refer to reference characters 10 and 10-1) describedhereinabove, the remaining configuration thereof is similar to that inthe embodiments described above. In particular, the core has a patternformed integrally thereon which functions as an input waveguide 1, aninput slab 2, an output slab 4, and output waveguides 5 similar to thoseof the embodiments described hereinabove in addition to the channelwaveguides 3-2 which have a characteristic unique to the thirdembodiment.

[0143] In particular, as shown in FIG. 10, an end portion 6-2 of each ofthe channel waveguides 3-2 has a pair of primary coupling portions 610including four primary branching connection branches 611 for receivingwavelength division multiplexed light from the input slab 2 and twoprimary merging portions 612 for optically coupling the wavelengthdivision multiplexed lights from the primary branching connectionbranches 611, and a secondary coupling portion 620 including twosecondary branching connection branches 621 for receiving the wavelengthdivision multiplexed lights coupled by the primary coupling portions 610and a secondary merging portion 622 for optically coupling thewavelength division multiplexed lights from the secondary branchingconnection branches 621, both formed integrally with each other.

[0144] In particular, in the MUX/DEMUX 10-2 in the third embodiment,each of the channel waveguides 3-2 is formed integrally with the inputslab 2 at four nodes. In other words, each of the primary couplingportions 610 and the secondary coupling portion 620 is formed as acoupling waveguide for optically coupling a plurality of wavelengthdivision multiplexed lights and propagating the resulting wavelengthdivision multiplexed light, and the primary coupling portions 610 andthe secondary coupling portion 620 each serving as a coupling waveguideare connected in tandem like a tree of two stages.

[0145] The four primary branching connection branches 611 of each of thechannel waveguides 3-2 are formed such that a center axis 31 (refer toFIG. 10) thereof at the end portion 6 of the channel waveguide 3-2adjacent the input slab 2 passes the center of a gap portion G2positioned between two primary branching connection branches 611 and anextension line of the center axis 31 passes an incoming light diffusioncenter 21 (refer to FIG. 9). In other words, the center axis 31 of theend portion 6-2 of each of the channel waveguides 3-2 coincides with theoptical axis of incoming light.

[0146] It is to be noted that, in FIG. 10, the width of the primarybranching connection branches 611 can be set to approximately 7 μm, andthe distance dc11 between the primary branching connection branches 611can be set to approximately 16 μm.

[0147] In particular, in the channel waveguide 3-2 shown in FIGS. 9 and10, since the primary branching connection branches 611 and the primarymerging portions 612 as branching connection branches are each formedfrom a waveguide of a fixed width of, for example, approximately 7 μm sothat higher-order mode light may be cut off, for example, if incominglight enters the primary branching connection branches 611, then sinceonly the 0th mode light is permitted to propagate in the primarybranching connection branches 611, no loss occurs there.

[0148] Similarly, while the secondary branching connection branches 621of the secondary coupling portion 620 propagate only wavelength divisionmultiplexed lights from the primary coupling portions 610, since each ofthe secondary branching connection branches 621 is formed from awaveguide of a fixed width of, for example, approximately 27 μm, itpropagates only the 0th mode light. It is to be noted that each of theprimary merging portions 612 and the secondary merging portion 622 isformed, similarly to the corresponding portions (refer to referencenumeral 69 in FIGS. 1 to 5) in the first embodiment describedhereinabove, with a width with which higher-order mode (first-ordermode) lights excited from wavelength division multiplexed lights fromthe primary branching connection branches 611 and 621 on the upstreamside can cancel each other.

[0149] With the wavelength division demultiplexing apparatus accordingto the third embodiment of the present invention, since the distancebetween the nodes 24 between the input slab 2 and the channel waveguides3 can be further reduced, the amount of loss caused by the gap portions(such portions as that denoted by reference numeral 123 in FIG. 32(c))can be reduced. In addition, the channel waveguide distance betweenconnection portions 44 between the output slab 4 and the channelwaveguides 3-2 can be increased when compared with that in theembodiments described hereinabove, and the interference of light at theend portions 7 of the channel waveguides 3-2 can be further suppressed.Thus, the wavelength division demultiplexing apparatus is effective forprevention of interference (coupling) of light where waveguides having acomparatively small refractive index difference with which interference(coupling) of light is likely to occur.

[0150] Further, while, in the third embodiment described above, the endportion 6-2 of each of the channel waveguides 3-2 has primary couplingportions 610 and a secondary coupling portion 620 connected in tandem ina tree-like configuration of two stages, according to the presentinvention, the end portion 6-2 is not limited to the specificconfiguration, and the end portion 6-2 may otherwise be configured usinga configuration similar to the configuration of the primary couplingportions 610 and the secondary coupling portion 620 described above ascoupling waveguides such that such coupling portions are connected intandem in a tree-like configuration of more than two stages.

[0151] Further, in the MUX/DEMUX 10-2 which functions as a wavelengthdivision demultiplexing apparatus according to the third embodimentdescribed above, for example, each of the primary branching connectionbranches 611 may be formed such that a center axis 32 c thereofcoincides with the optical axis of incoming light from the incominglight diffusion center 21 as shown in FIG. 11. The configuration justdescribed provides advantages similar to those of the configurationdescribed above with reference to FIG. 11.

[0152] (d) Fourth Embodiment

[0153] FIGS. 12 to 14 show a fourth embodiment of the present invention.More particularly, FIG. 12 schematically shows principal components of aMUX/DEMUX 10-3 which functions as a wavelength division demultiplexingapparatus according to the fourth embodiment of the present inventionand particularly shows a pattern of a core of optical waveguide deviceswhich compose the MUX/DEMUX 10-3. FIG. 13 schematically shows part of aninput waveguide 1, an input slab 2 and channel waveguides 3-3 in anenlarged scale, and FIG. 14 schematically shows an end portion 6-3 ofone of the channel waveguides 3-3 adjacent the input slab 2.

[0154] Also in the MUX/DEMUX 10-3 according to the fourth embodiment,similarly as in the embodiments described hereinabove, a core is formedsuch that upper, lower and left, right portions thereof are surroundedby an under clad or an over clad so that light can be propagated in aconfined state in the core.

[0155] While the MUX/DEMUX 10-3 according to the fourth embodiment isdifferent in configuration of the channel waveguides 3-3 thereof fromthat in the first embodiment (refer to reference character 10) describedhereinabove, the remaining configuration thereof is similar to that inthe embodiments described above. In particular, the core has a patternformed integrally thereon which functions as an input waveguide 1, aninput slab 2, an output slab 4, and output waveguides 5 similar to thoseof the embodiments described hereinabove in addition to the channelwaveguides 3-3 which have a characteristic unique to the fourthembodiment.

[0156] Each of the channel waveguides 3-3 in the fourth embodiment has acharacteristic core pattern at a portion thereof in the proximity of aportion at which it is optically connected to the input slab 2, that is,an end portion 6-3 thereof adjacent the input slab 2.

[0157] In particular, the end portion 6-3 of each of the channelwaveguides 3-3 of the MUX/DEMUX 10-3 (refer to FIG. 12 or 13) of thefourth embodiment has, as shown in FIG. 14, a pair of tapering portions65 p each having a width which is a small width W_(p) at the node 24 tothe input slab 2 and increases up to W_(o) as the distance from theinput slab 2 increases. Here, if the width W_(o) is set to 7 μm and thewidth W_(p) is set to 2 μm, then the amount of connection loss betweenthe input slab 2 and the channel waveguide 3 can be reduced whencompared with that in the first or second embodiment.

[0158] Thus, each of the branching connection branches 65 has a taperingportion 65 p having a pattern wherein the width decreases in a taperingfashion as the distance to the input slab 2 decreases. In other words,the channel waveguides 3-3 each having two branching connection branches65 each having a tapering portion 65 p and a merging portion 69 at theend portion 6-3 thereof are formed integrally with the input slab 2 suchthat the input slab 2 and the channel waveguides 3-3 are opticallyconnected to each other.

[0159] It is to be noted that the branching connection branches 65 areformed such that, as shown in FIG. 14, a center axis 33 b at the endportion 6-3 of each of the channel waveguides 3-3 passes the center of agap portion G1 positioned between the two branching connection branches65 and an extension line of the center axis 33 b passes an incominglight diffusion center 21.

[0160] Now, the reason why the connection loss between the input slab 2and the channel waveguides 3 in the MUX/DEMUX of the fourth embodimentdecreases is described through comparison between FIGS. 15(a) and 15(b).

[0161]FIG. 15(a) illustrates operation of an input side connectionportion 6-3 of the MUX/DEMUX 10-3 (refer to FIG. 12) according to thefourth embodiment, and FIG. 15(b) illustrates operation of an input sideconnection portion 6 of the MUX/DEMUX 10 (refer to FIG. 1) in the firstembodiment.

[0162] Since the core width of each of the branching connection branches61 in FIG. 15(b) is 7 μm fixed, the intensity distribution 81 ofelectric field at an end portion D1 of the branching connection branch61 is same as the intensity distribution 84 a of electric field atanother portion D2 of the branching connection branch 61. In thisinstance, the coupling efficiency between the input slab 2 and thechannel waveguide 3 of the MUX/DEMUX 10 of the first embodiment is equalto an integration over the area of overlap between the normalizedoptical fields 24 and 81 of the intensity 24 of electric field of lightimmediately before incoming to the channel waveguide 3 and the intensity24 of electric field of 0th-order mode light propagating in thebranching connection branches of the channel waveguide 3. It is to benoted that this calculation method is disclosed, for example, in “IEEEJOURNAL OF QUANTUM ELECTRONICS, VOL. 28 No. 12, p.2729 (1992)”.

[0163] Since the intensity 24 of electric field is an intensitydistribution of electric field of light diffused by the input slab 2,the width of it is great. In contrast, since the intensity 81 ofelectric field is a mode excited in the core 51 whose width is 7 μm, thewidth of it is small.

[0164] In this manner, as the ratio in width between intensities ofelectric fields to be coupled increases, the coupling loss increases.

[0165] In contrast, since the end portion of each of the branchingconnection branches 61 employed in the MUX/DEMUX 10-3 of the fourthembodiment is small in width, the waveguide mode excited at the endportion is great in width. As a result, the ratio in width between theintensity 24 of electric field and the intensity distribution 82 ofelectric field becomes small, and the coupling loss is reduced.

[0166] On the other hand, where a wavelength division demultiplexingapparatus includes the branching connection branches 65 each having thetapering portion 65 p as in the fourth embodiment, incoming lightemitted from the input waveguide 1 and propagating in the input slab 2until it comes to the slab boundary line 22 exhibits such an intensitydistribution 8 of electric field as seen in FIG. 15(a). Further,waveguide mode light excited at an end portion D1 of each of thebranching connection branches 65 has such an intensity distribution 83of electric field, and waveguide mode light excited at a portion D2 ofthe branching connection branch 65 at which the tapering portion 65 pcomes to an end has such an intensity distribution 84 of electric fieldas seen in FIG. 15(a).

[0167] It is to be noted that, while, in FIG. 15(b), each of thebranching connection branches 65 is formed such that the width thereofincreases as the distance from the end portion D1 increases along thetapering portion 65 p, the waveguide width W_(o) at the portion D2 atwhich the increase of the width along the tapering portion 65 p comes toan end is set to approximately 7 μm, the waveguide width W_(p) at theend portion D1 to approximately 2 μm, and the length Lp of the taperingportion 65 p to approximately 2.5 mm.

[0168] Here, since each of the channel waveguides in the presentembodiment is formed from a single mode waveguide, it has acharacteristic that the electric field distribution expands if thewaveguide width becomes smaller than approximately {fraction (1/2)}. Inparticular, as seen from FIGS. 15(a) and 15(b), the intensitydistribution 83 of electric field at the end portion D1 of the branchingconnection branch 65 is wider than the intensity distribution 81 ofelectric field at a corresponding portion of the branching connectionbranch 61 (that is, the waveform of the intensity distribution becomesflattened).

[0169] It is to be noted that the intensity distributions It is to benoted that the intensity distributions 81 and 84 a of electric field atthe end portions D1 and D2 of the branching connection branch 61 and theintensity distribution 84 of electric field at the end portion D2 of thebranching connection branch 65 are same in width and shape since thecore widths of the portions at which excitation occurs are equal to oneanother.

[0170] Here, the coupling efficiencies of light of the branchingconnection branches 61 and 65 on the slab boundary line 22 are equal tosuperposition integration values of the intensity distribution 8 ofelectric field of incoming light and the intensity distributions 81 and83 of electric field of waveguide mode lights excited in the waveguide[refer to, for example, Kenji Kono, “Foundations and Applications ofOptical Coupling Systems for Optical Devices”, Gendai Kogaku-Sha, p31,expression (3.1-7)]. Through the supervision integration, a result isobtained that the configuration of the branching connection branches 65exhibits a higher coupling efficiency than the configuration of thebranching connection branches 61.

[0171] Accordingly, where the branching connection branches 65 eachhaving the tapering portion 65 p are formed, the connection loss betweenthe input slab 2 and the channel waveguides 3-3 on the slab boundaryline 22 can be reduced when compared with that where branchingconnection branches are formed without having the tapering portion 65 p.

[0172] In this manner, with the wavelength division demultiplexingapparatus according to the fourth embodiment of the present invention,since a portion of each of the channel waveguides 3-3 in the proximityof a portion at which it is optically connected to the input slab 2 hasintegrally formed thereon two branching connection branches 65 and amerging portion 69 for optically coupling wavelength divisionmultiplexed lights from the branching connection branches 65, similaradvantages to those of the first embodiment described above areachieved. Further, since each of the branching connection branches 65 isformed so as to have, at the tapering portion 65 p thereof, a widthwhich decreases in a tapering fashion as the distance to the input slab2 decreases, the connection loss between the input slab 2 and each ofthe channel waveguides 3 can be reduced when compared with that whereotherwise the branching connection branches 65 do not have the taperingportion 65 p.

[0173] It is to be noted that, while, in the fourth embodiment describedabove, the center axis 33 b of the end portion 6-3 of each of thechannel waveguides 3-3 coincides with the optical axis of incominglight, according to the present invention, each of the branchingconnection branches 65 may be formed such that a center axis 33 athereof is disposed on an extension line from the diffusion center 21.

[0174] In other words, the branching connection branches 65 of thechannel waveguides 3-3 shown in FIG. 16 are disposed such that thecenter axis 33 a of each of them intersects perpendicularly with atangential line to the arc of the slab boundary line 22. Where thebranching connection branches 65 are formed in this manner, the couplingloss can be further reduced and the incoming efficiency of wavelengthdivision multiplexed light can be further raised when compared withthose of the embodiment described hereinabove.

[0175] Operation of the MUX/DEMUX 10-3 according to the presentinvention and operation of a conventional MUX/DEMUX are described incomparison with each other with reference to FIG. 17. FIG. 17 shows acore pattern corresponding to the manner of propagation of light. Here,when laser light is inputted from the input waveguide 101, the lightpropagates into the input slab 102 and one waveguide (channel waveguide103-1) having characteristics similar to those of the channel waveguide103-1. Further, the color varies like deep blue→yellow→deep red (notshown) in proportion to the intensity of propagated light.

[0176] When laser light is inputted to the input waveguide 101, thelight is propagated into the channel waveguide 103-1 through the inputslab 102. Thereupon, at a portion of the channel waveguide 103-1 atwhich the tapering connection branch 162 has a reduced width,higher-order mode is radiated.

[0177] Then, when laser light is inputted from the input waveguide 1shown in FIG. 12, the light propagates into the input slab 2 and onewaveguide (channel waveguide 3-3) having characteristics similar tothose of the channel waveguide 3-3. Thereupon, the color varies likedeep blue→yellow→deep red in proportion to the intensity of propagatedlight. FIG. 18 shows a core pattern corresponding to the manner ofpropagation of light.

[0178] In the configuration shown in FIG. 18, light propagating in thechannel waveguide 3-3 through the input slab 2 is radiated but by a muchreduced light amount to the outside of the core from the branchingconnection branch 65 when compared with light propagating in the channelwaveguide 103-1, and the light loss can be reduced significantly. Inthis instance, the light loss in the configuration shown in FIG. 17 isapproximately −16.4 dB and the light loss in the configuration of FIG.18 is approximately −14.7 dB, and a loss reduction effect byapproximately 1.7 dB is obtained.

[0179] According to a simulation, the wavelength division demultiplexingapparatus 10-3 according to the fourth embodiment exhibited a lossreduction effect of 1.7 dB when compared with that of a conventionalwavelength division demultiplexing apparatus (refer to FIG. 28).

[0180] (e) Fifth Embodiment

[0181] FIGS. 19 to 21 show a fifth embodiment of the present invention.More particularly, FIG. 19 schematically shows principal components of aMUX/DEMUX 10-4 which functions as a wavelength division demultiplexingapparatus according to the fourth embodiment of the present inventionand particularly shows a pattern of a core of optical waveguide deviceswhich compose the wavelength division multiplexing and demultiplexingapparatus 10-4. FIG. 20 schematically shows part of an input waveguide1, an input slab 2 and channel waveguides 3-4 in an enlarged scale, andFIG. 21 schematically shows an end portion 6-4 of one of the channelwaveguides 3-4 adjacent the input slab 2.

[0182] Also in the MUX/DEMUX 10-4 according to the fifth embodiment,similarly as in the embodiments described hereinabove, a core is formedsuch that upper, lower and left, right portions thereof are surroundedby an under clad or an over clad so that light can be propagated in aconfined state in the core.

[0183] While the MUX/DEMUX 10-4 according to the fifth embodiment isdifferent in configuration of the channel waveguides 3-4 thereof fromthat in the fourth embodiment (refer to reference character 10-3)described hereinabove, the remaining configuration thereof is similar tothat in the embodiments described above. In particular, the core has apattern formed integrally thereon which functions as an input waveguide1, an input slab 2, an output slab 4, and output waveguides 5 similar tothose the channel waveguides 3-4 which have a characteristic unique tothe fourth embodiment.

[0184] Each of the channel waveguides 3-4 in the fifth embodiment has acharacteristic core pattern at a portion thereof in the proximity of aportion at which it is optically connected to the input slab 2, that is,an end portion 6-4 thereof adjacent the input slab 2.

[0185] In particular, the end portion 6-4 of each of the channelwaveguides 3-4 of the MUX/DEMUX 10-4 (refer to FIG. 19 or 20) accordingto the fifth embodiment has, as shown in FIG. 21, a pair of portions 66s having a width which is a small width W_(p) at the node 24 to theinput slab 2 and is fixed over a fixed distance from the input slab 2,and a pair of tapering portions 66 p each having a width which increasesup to W_(o) as the distance from the input slab 2 increases.

[0186] Here, if the width W_(o) is set to 7 μm and the width W_(p) isset to 2 μm, then the amount of connection loss between the input slab 2and the channel waveguide 3 can be reduced when compared with that inthe fourth embodiment.

[0187] The tapering portion 66 p has a width which decreases in atapering fashion as the distance to the input slab 2 from the mergingportion 69 side decreases. The narrow fixed width waveguide portion 66 shas a substantially fixed width substantially equal to the width of aminimum width portion of the tapering portion 66 p and opticallyconnects the input slab 2 and the tapering portion 66 p to each other.the input slab 2 and the tapering portion 66 p to each other.

[0188] It is to be noted that a center axis 31 of the end portion 6-4 ofeach of the channel waveguides 3-4 passes the center of a gap portion G1positioned between the two branching connection branches 66 and anextension line of the center axis 33 b passes the incoming lightdiffusion center 21. In other words, the center axis 33 b of the endportion 6-4 of each of the channel waveguides 3-4 coincides with theoptical axis of incoming light to the channel waveguide 3.

[0189] Further, in the wavelength division multiplexing anddemultiplexing apparatus 10-4 according to the fifth embodiment, sinceeach of the branching connection branches 66 has a tapering portion 66 pand a narrow fixed width waveguide portion 66 s, the connection lossbetween the input slab 2 and the channel waveguides 3-4 can be furtherreduced when compared with that of the wavelength division multiplexingand demultiplexing apparatus 10-3 which includes the branchingconnection branches 65 as in the fourth embodiment.

[0190] Here, the reason why the connection loss between the input slab 2and the channel waveguides 3 in the MUX/DEMUX 10-4 according to thefifth embodiment decreases is described through comparison between FIGS.22(a) and 22(b).

[0191]FIG. 22(a) illustrates operation of an input side connectionportion 6-3 of the MUX/DEMUX according to the fifth embodiment, and FIG.22(b) illustrates operation of an input side connection portion 6 of theMUX/DEMUX in the fourth embodiment.

[0192] Incoming light emitted from the input waveguide 1 and propagatingin the input slab 2 until it comes to the slab boundary line 22 exhibitsan intensity distribution 8 of electric field while waveguide mode lightexcited at the end portion D1 of the narrow fixed width waveguideportion 66 s has an intensity distribution 83 of electric field.Further, waveguide mode light excited at the portion D2 at which thetapering portion 65 p of the branching connection branch 65 terminalshas an intensity distribution 84 of electric field.

[0193] Here, in the case of the branching connection branch 65 employedin the MUX/DEMUX 10 of the first embodiment, it exhibits its minimumwidth only just at the portion thereof on the slab boundary line 22 asseen in FIG. 22(b). In order for light propagating in the branchingconnection branch 65 to have an intensity distribution of electric fieldcorresponding to the core width of 2 μm, it is necessary for the lightto propagate in a core over a distance longer than at least thewavelength thereof. Here, it is practically necessary for the light topropagate in a core of a length greater than ten times the wavelengththereof.

[0194] However, since the length of the portion of the branchingconnection branch 65 shown in FIG. 22(b) whose width is 2 μm isinfinitely proximate to 0 and shorter than the required length, thewidth of the intensity distribution of electric field of the modeexcited in the proximity of the input slab 2 is smaller than the widthof the intensity distribution of electric field excited in the corehaving a core width of 2 μm.

[0195] As a result, the coupling loss becomes greater than the couplingloss expected where the width of the branching connection branch 65 is 2μm.

[0196] In contrast, in the case of FIG. 22(a) corresponding to the inputside connection portion 6-4 in the fifth embodiment, the core width ofthe end portion of the branching connection branch 65 can be set to 2 μmfixed over a length greater than ten times the wavelength of the light.Accordingly, where the width of the branching connection branch 65 is 2μm, the coupling loss can be reduced to a value expected therefore.

[0197] On the other hand, where a wavelength division demultiplexingapparatus includes the branching connection branches 66 each having thetapering portion 66 p and the narrow fixed width waveguide portion 66 sas in the fifth embodiment, incoming light emitted from the inputwaveguide 1 and propagating in the input slab 2 until it comes to theslab boundary line 22 exhibits such an intensity distribution 8 ofelectric field as seen in FIG. 22(a). Further, waveguide mode lightexcited at an end portion D1 of each of the branching connectionbranches 66 has such an intensity distribution 83 a of electric field,and waveguide mode light excited at a portion D2 of the branchingconnection branch 66 at which the tapering portion 66 p comes to an endhas such an intensity distribution 84 of electric field as seen in FIG.22(a).

[0198] It is to be noted that, while, in FIG. 22(a), the waveguide widthW0 at the portion D2 at which the increase of the width along thetapering portion 66 p comes to an end can be set to approximately 7 μm,the waveguide width Wp at the end portion D1 to approximately 2 μm, thelength Lp of the tapering portion 66 p to approximately 800 μm, and thelength Ls of the narrow fixed width waveguide portion 66 s toapproximately 200 μm.

[0199] In the tapering portion 65 p of the branching connection branch65 and the tapering portion 66 p of the branching connection branch 66,the electric field distribution guided in the waveguide variescontinuously in accordance with the variation of the core width of thewaveguide. Therefore, if the narrow fixed width waveguide portion 66 sis provided such that the tapering portion 66 p is formed with a smallerlength than that of the tapering portion 65 p, then the extent ofelectric field mode light (waveguide mode light) excited in the endportion D1 is greater than that in the case of the branching connectionbranch 65.

[0200] It is to be noted that, while, in the fifth embodiment describedabove, the center axis 33 b of the end portion 6-4 of each of thechannel waveguides 3-4 coincides with the optical axis of incominglight, according to the present invention, each of the branchingconnection branches 66 a maybe formed such that a center axis 33 athereof is disposed on an extension line from the incoming lightdiffusion center 21.

[0201] In other words, the branching connection branches 66 a of thechannel waveguides 3-4 a shown in FIG. 23 are disposed such that thecenter axis 33 a of each of them intersects perpendicularly with atangential line to the arc of the slab boundary line 22. Where thebranching connection branches 66 a are formed in this manner, thecoupling loss can be further reduced and the incoming efficiency ofwavelength division multiplexed light can be further raised whencompared with those of the embodiment described hereinabove.

[0202] (f) Others

[0203] In the MUX/DEMUX 10-2 which functions as a MUX/DEMUX according tothe third embodiment described hereinabove, the primary branchingconnection branches 611 of the end portion 6-2 of each of the channelwaveguides 3-2 are formed such that they have center axis 32 b extendingin parallel to each other and have a fixed width. According to thepresent invention, however, the end portion 6-2 of each of the channelwaveguides 3-2 is not limited to the specific configuration. Forexample, such configurations of end portions 6-21 to 6-24 of each ofchannel waveguides 3-21 to 3-24 as shown in FIG. 24 to 27 can be used.

[0204] In particular, as seen in FIG. 24, the end portion 6-21 of eachof the channel waveguides 3-21 may have primary branching connectionbranches 651 each having a tapering portion 65 p similar to thatdescribed hereinabove with reference to FIGS. 12 to 14. Thisconfiguration provides similar advantages to those of the fourthembodiment described hereinabove.

[0205] Further, for example, as shown in FIG. 25, each of the primarybranching connection branches 651 of the end portion 6-22 of each of thechannel waveguides 3-22 may be configured such that it has a taperingportion 65 p similar to that in the fourth embodiment (refer to FIGS. 12to 14) and has a center axis 32 c which coincides with the optical axisof incoming light. This configuration provides advantages similar tothose of the fourth embodiment. Further, the coupling loss between theinput slab 2 and the channel waveguides 3-2 can be further reducedsimilarly as in the case of FIG. 11.

[0206] On the other hand, for example, as seen in FIG. 26, each of theprimary branching connection branches 661 of the end portion 6-23 ofeach of the channel waveguides 3-23 may have a tapering portion 66 p anda narrow fixed width waveguide portion 66 s similar to those in thefifth embodiment (refer to FIGS. 19 to 21). Where this configuration isemployed, advantages similar to those in the fifth embodiment describedabove can be achieved.

[0207] Furthermore, for example, as seen in FIG. 27, each of the primarybranching connection branches 661 of the end portion 6-24 of each of thechannel waveguides 3-24 may be configured such that it has a taperingportion 66 p and a narrow fixed width waveguide portion 66 s similar tothose in the fifth embodiment (refer to FIGS. 19 to 21) and has a centeraxis 32 coincident with the optical axis of incoming light. Thisconfiguration provides advantages similar to those of the fifthembodiment. Further, the coupling loss between the input slab 2 and thechannel waveguides 3-2 can be further reduced similarly as in the caseof FIG. 11.

[0208] Further, the island-shaped formation region 34 in the secondembodiment described hereinabove can naturally be applied to the channelwaveguides in the other embodiments than the second embodiment.

[0209] The present invention is not limited to the embodimentsspecifically described above, and variations and modifications can bemade without departing from the scope of the present invention.

What is claimed is:
 1. An apparatus comprising: first and secondwaveguides extending from an optical propagating member and mergingtogether into a same waveguide, to guide the light from the opticalpropagating member to said same waveguide, said same waveguide having awidth narrower than a width of the optical propagating member.
 2. Anapparatus as in claim 1, wherein each of the first and second waveguideshave a width at which higher-order mode light is cut off, and a mergingportion at which the first and second waveguides merge together intosaid same waveguide has a width at which higher-order mode light isexcited.
 3. An apparatus as in claim 1, wherein the first and secondwaveguides have the same width, and a merging portion at which the firstand second waveguides merge together into said same waveguide has awidth at least equal to the widths of the first and second waveguidesadded together.
 4. An apparatus as in claim 1, wherein each of the firstand second waveguides has a width which tapers as a distance toward theoptical propagation member decreases.
 5. An apparatus as in claim 1,wherein widths of the first and second waveguides are constant forlengths of the first and second waveguides, respectively.
 6. Anapparatus as in claim 1, wherein the apparatus is an array waveguidegrating.
 7. An apparatus as in claim 2, wherein the apparatus is anarray waveguide grating.
 8. An apparatus as in claim 3, wherein theapparatus is an array waveguide grating.
 9. An apparatus as in claim 4,wherein the apparatus is an array waveguide grating.
 10. An apparatus asin claim 5, wherein the apparatus is an array waveguide grating.
 11. Anapparatus comprising: an input slab diffusing light input to the inputslab; a channel waveguide having a width narrower than a width of theinput slab; and first and second branching waveguides branching from theinput slab and merging together into the channel waveguide, to guide thediffused light from the input slab to the channel waveguide.
 12. Anapparatus as in claim 11, wherein each of the first and second branchingwaveguides have a width at which higher-order mode light is cut off, anda merging portion at which the first and second branching waveguidesmerge together into the channel waveguide has a width at whichhigher-order mode light is excited.
 13. An apparatus as in claim 11,wherein the first and second branching waveguides have the same width,and a merging portion at which the first and second branching waveguidesmerge together into the channel waveguide has a width at least equal tothe widths of the first and second branching waveguides added together.14. An apparatus as in claim 11, wherein each of the first and secondbranching waveguides has a width which tapers as a distance toward theinput slab decreases.
 15. An apparatus as in claim 11, wherein widths ofthe first and second branching waveguides are constant for lengths ofthe first and second branching waveguides, respectively.
 16. Anapparatus as in claim 11, wherein the apparatus is an array waveguidegrating.
 17. An apparatus as in claim 12, wherein the apparatus is anarray waveguide grating.
 18. An apparatus as in claim 13, wherein theapparatus is an array waveguide grating.
 19. An apparatus as in claim14, wherein the apparatus is an array waveguide grating.
 20. Anapparatus as in claim 15, wherein the apparatus is an array waveguidegrating.